Reducing underfill keep out zone on substrate used in electronic device processing

ABSTRACT

Electronic devices and methods for fabricating electronic devices are described. One method includes providing a substrate with a die attach area, and forming a layer on the substrate outside of the die attach area. The layer may be formed from a fluoropolymer material. The method also includes coupling a die to the substrate in the die attach area, wherein a gap remains between the die and the die attach area. The method also includes placing an underfill material in the gap and adjacent to the layer on the substrate. Examples of fluoropolymer materials which may be used include polytetrafluoroethylene (PTFE) and perfluoroalkoxy polymer resin (PFA). Other embodiments are described and claimed.

This application is a divisional of U.S. patent application Ser. No.11/617,640 now U.S. Pat. No. 7,875,503 filed Dec. 28, 2006, which ishereby incorporated by reference in its entirety.

RELATED ART

Integrated circuits may be formed on semiconductor wafers made ofmaterials such as silicon. The semiconductor wafers are processed toform various electronic devices. The wafers are diced into semiconductorchips, which may then be attached to a substrate using a variety ofknown methods. For example, bonding pads formed on the chip may beelectrically coupled to the substrate using a variety of connectionapproaches, including, but not limited to, BGA (ball grid array), PGA(pin grid array), and LGA (land grid array). Such approaches couple thedie to a substrate with a small gap therebetween.

FIGS. 1-3 illustrate certain operations in a conventional die attachprocess. As illustrated in FIG. 1, a die 10 is mounted on a substrate12. In this example, the die 10 is mounted to the substrate 12 using aconventional BGA arrangement in a flip chip configuration, using amethod known as a C4 process, in which solder bumps 14 are locatedbetween the die 10 and substrate 12. In a C4 process, the solder bumps14 may be placed on pads on the active side of the die 10, on thesubstrate 12, or on both the die 10 and substrate 12. The solder bumps14 are then melted and permitted to flow, to ensure that each bump fullywets the pad it was formed on. A flux may then be applied to at leastone of the surfaces to be joined and the surfaces on the die 10 andsubstrate 12 are brought into electrical contact through the solderbumps 14. The flux acts to isolate the solder from the atmosphere andprovides an adhesive force which acts to hold the die 10 and substrate12 together. A second reflow operation is then carried out by heatingthe die 10 and substrate 12 to a temperature greater than the meltingpoint of the solder, and a solder connection is made between the die 10pads and the substrate 12 pads. The joined package is then cooled andthe solder solidified. Excess flux residue may be removed in a defluxingoperation, which may include chemical rinsing and heating operations. Amaterial such as a polymer is then typically applied between the chipand substrate as an underfill encapsulant.

As illustrated in FIG. 1, a dispenser 16 such as a needle is positionedadjacent to the die 10, and an underfill material 18 is dispensed on thesubstrate 12 next to the die attach area (the area of the substrate 12that is or will be covered by the die 10). The underfill material 18 istypically a polymer, for example, an epoxy. With the application of heatto the substrate and/or die, the underfill material 18 may be made toflow between the die 10 and substrate 12, using capillary action. Whenformed from a material such as a polymer epoxy, the underfill material18 is then typically cured, to harden the polymer. The cured underfillmaterial 18 surrounds the solder bumps 14 and acts to protect the bumpsand connection between the die 10 and substrate 12, as well as supportthe die 10 on the substrate 12.

As seen in FIGS. 2-3, part of the underfill material 18 extends on thesubstrate beyond the die attach area to the sides of the die 10, andincludes a tongue region 24, which is on the side or sides where theunderfill material 18 was dispensed, and a side spread region 26. Theunderfill material 18 typically extends a distance up the side surfacesof the die 10. As seen in the example illustrated in FIG. 3, the tongueregion 24 extends away from the side surface 10 a of the die 10, andside spread regions 26, 28, and 30 extend away from the side surfaces 10b, 10 c, and 10 d of the die 10.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described by way of example, with reference to theaccompanying drawings, which are not drawn to scale, wherein:

FIG. 1 illustrates a side cross-sectional view of a conventionalprocessing operation in which an underfill material is dispensed on asubstrate;

FIG. 2 illustrates a side cross-sectional view of a conventionalprocessing operation in which an underfill material is positionedbetween the die and substrate and extends on the substrate beyond thearea covered by the die;

FIG. 3 illustrates a top view of a conventional processing operationincluding an underfill material extending to a tongue region and sidespread regions;

FIGS. 4-7 illustrate views of processing operations including thepositioning of a low surface energy material layer on a substrate andthe positioning of an underfill material in a tongue region and in sidespread regions, in accordance with certain embodiments.

FIG. 8-11 illustrate views of processing operations including thepositioning of a low surface energy material layer on a substrate andthe positioning of the underfill material in a tongue region, inaccordance with certain embodiments;

FIG. 12 illustrates a top view showing no underfill material spreadoutwards on the substrate to the sides of the die, in accordance withcertain embodiments.

FIG. 13 illustrates a flow chart of an assembly process, in accordancewith certain embodiments; and;

FIG. 14 illustrates an electronic system arrangement in whichembodiments may find application.

DETAILED DESCRIPTION

The outward flow from the die attach area to create the tongue and sidespread regions as illustrated in FIGS. 2-3 can lead to certain problemsdue to the large surface area that becomes covered with the underfillmaterial. Such problems may relate to interactions between the underfillmaterial and other structures, for example, when the substrate is smallas in ultra small form factors, and when die side capacitors are mountedon the die side of the substrate. As a result, the underfill keep outzone on the substrate (the area where other structures are kept out dueto the spread of the underfill material beyond the die attach area) maybe undesirably large.

Certain embodiments relate to the formation of electronic assemblies, inwhich the flow of underfill material outside of the die attach area iscontrolled. Certain embodiments may include electronic assemblies.Certain embodiments may also include methods for forming electronicassemblies.

FIGS. 4-7 illustrate views of an electronic assembly during severalstages of processing, in accordance with certain embodiments. As seen inthe top view of FIG. 4, a low surface energy material layer 120 isformed on a portion of the surface of substrate 112. A die attach area115 is bounded by the dotted lines in FIG. 4. The die attach area 115 isthe area on the substrate that will be covered by the die 110 (whenviewed from above the die) when the die 110 is coupled to the substrate112. The region 117 (outside of the die attach area 115 bounded by thedotted line but inside of the low surface energy material layer 120) iswhere a tongue region 124 and side spread regions 126, 128, and 130 willbe formed (see FIG. 7) on the substrate during subsequent processing.

As seen in FIG. 5, a dispenser 116 such as a needle is positionedadjacent to the die 110, which is coupled to the substrate 112 throughsolder bumps 114. An underfill material 118 is dispensed on thesubstrate 112, next to the die attach area. The underfill material 118will flow between the die 110 and substrate 112, for example, throughthe use of heat induced capillary action. The substrate 112 alsoincludes the low surface energy material layer 120 formed thereon. Thelow surface energy material layer 120 is positioned outside of the dieattach area 115. The low surface energy material layer 120 is formedfrom material having a low surface energy and having the ability toinhibit the flow of the underfill material 118. As a result, when theunderfill material is flowed between the die and substrate (for example,through the use of heat induced capillary action), the low surfaceenergy material layer 120 inhibits the continued outward flow of theunderfill material 118.

Examples of low surface energy materials include, but are not limitedto, fluoropolymer materials (polymer which have a fluorinated carbon).One example is a family of materials that are sold under the trade nameTeflon, (available from DuPont). One material sold under the trade nameTeflon is known as PTFE (polytetrafluoroethylene). A basic PTFE chemicalstructure is set forth in Table 1 below, where C is carbon, F isfluorine, and n is a variable. Another material sold under the tradename Teflon is known as PFA (perfluoroalkoxy polymer resin). A basic PFAstructure is set forth in Table 1 below, wherein C is carbon, F isfluorine, O is oxygen, and m and n are variables.

TABLE 1 Low Surface Energy Materials Structure Name

PTFE

PFA

PTFE and PFA have non-stick properties, have minimal surface reactiveproperties, and have a melting point of about 300° C., which isgenerally not detrimental to electronic assembly processes. Due to thelow surface energy, these materials will inhibit the underfill materialfrom wetting their surface and thus inhibit the underfill from stickingthereto. As a result, the spreading of the underfill material over thesubstrate surface can be controlled because it will be inhibited fromflowing over the low surface energy material. The low surface energymaterial may be positioned to permit whatever amount of spreading of theunderfill material is desired. For example, in certain applications, itmay be useful to have the underfill material spread a small, uniformdistance outward from the die attach area, for protection. In otherapplications it may be useful to minimize or have no underfill spreadbeyond the die attach area, or to provide for different amounts ofunderfill spread on different sides of the die attach area.

As illustrated in the embodiment shown in FIGS. 4-7, the low surfaceenergy material layer 120 is positioned a first distance outside of thedie attach area 115 on one side of the die attach area and is positioneda further distance outside of the die attach area on the other threesides of the die attach area 115. As a result there will be a larger(but uniform) spread on one side, which is in this embodiment the sidethat the underfill material 118 is dispensed on (the tongue region), asseen in FIG. 5. The other three sides outside of the die attach area 115will each have a small and uniform side spread region, as illustrated inFIG. 7.

FIG. 6 shows the underfill material 118 after it has flowed between thedie 110 and substrate 112. The underfill material 118 has not flowedover the low surface energy material layer 120. In certain embodiments,it is desired that the underfill material 118 spreads at least adistance up the side surfaces of the die 110. For instance, asillustrated in FIG. 6, the underfill material 118 spreads about half wayup the illustrated side surfaces 110 a and 110 c of the die 110.

As illustrated in FIG. 7, when viewed from above, such a design enablesthe underfill material to spread outward on the substrate 112 from thesides 110 a, 110 b, 110 c, and 110 d of the die 110. As illustrated inthe embodiment of FIG. 7, the tongue region 124 extending on thesubstrate 112 outward from die side 110 a is larger than the side spreadregions extending on the substrate 112 outward from the die sides 110 b,110 c, and 110 d. In this embodiment, the tongue region 124 and sidespread regions 126, 128, 130 extending on the substrate outward from thedie sides 110 b, 110 c, and 110 d, are substantially smaller than thoseillustrated in the conventional approach illustrated in FIG. 3.

It should be appreciated that the exact position and shape of the lowsurface energy material layer 120 may be varied from that shown in FIGS.4-7. Depending on factors including, for example, the size of thesubstrate and the presence of other structures on the substrate, theunderfill keep out zone can be configured to any desired shape and anydesired size outside of the die attach area (even down to zero on allsides), by controlling the location, shape and size of the low surfaceenergy material formed outside of the die attach area.

For example, in another embodiment, the low surface energy materiallayer may be positioned a distance outside of the die attach area on oneside only, as illustrated in FIGS. 8-11. FIG. 8 shows the low surfaceenergy material layer 220 positioned a distance outside of the dieattach area 215 on only one side of the die attach area 215, so that asmall region 217 outside of the die attach area 215 is not covered bythe low surface energy material layer 220. Region 217 is where theunderfill will flow to form a tongue region. FIG. 9 illustrates a die210 coupled to a substrate 212 through bumps 214. A dispenser 216dispenses an underfill material 218 onto the surface of the substrate212 adjacent to the die 210. FIG. 10 illustrates the flow of theunderfill material 218 under the die and into the region outside of thedie attach area 215 that is not covered with the low surface energymaterial layer 220.

FIG. 11 shows a top view of the position of the flowed underfillmaterial on the substrate 212. As seen in FIG. 11, the tongue region 224has a controlled and uniform shape and is substantially smaller thanthat illustrated in the conventional assembly illustrated in FIG. 3. Inaddition, as seen in FIG. 11, there is no side spread extending outwardon the substrate 112 from the sides 210 b, 210 c, and 210 d of the die210, unlike in the conventional assembly illustrated in FIG. 3, whichshows side spread regions 26, 28, and 30 extending outward on thesubstrate 12. Any underfill material that is spread up the sides 210 b,210 c, and 210 d of the die 210 (such as the underfill shown extendingup the side surface 210 c in FIG. 10), is not shown in FIG. 11.

Another embodiment includes positioning the low surface energy materiallayer so that there is no tongue and no side spread formed on thesubstrate on any of the sides of the die attach area. Such an embodimentis illustrated in the top view FIG. 12, which shows that when viewedfrom above, the low surface energy material layer 320 is positioned tothe edge of the die attach area, and thus there is no tongue region andno side spread regions extending outward on the substrate from the sidesurfaces 310 a, 310 b, 310 c, and 310 d of the die 310. It should alsobe appreciated that the die attach area may have multipleconfigurations, for example, more or less than 4 sides, depending on theshape of the die.

FIG. 13 is a flow chart showing a number of operations in accordancewith certain embodiments. Box 350 is forming a low surface energymaterial layer on a substrate, outside of a die attach area. Asdescribed in the embodiments above, the low surface energy material maybe positioned so that there will occur a desired amount of underfillmaterial spread on the substrate, ranging from zero spread on all sidesof the die attach area, to spread on all sides of the die attach area.Box 352 is coupling a die to the substrate in the die attach area,leaving a gap between the die and the substrate. In certain embodiments,the gap includes a plurality of solder bumps. Box 354 is dispensing anunderfill material adjacent to the die on the substrate. Box 356 isfilling the gap between the die and substrate with the underfillmaterial, using a method such as capillary action. Where a material suchas a polymer epoxy is used as the underfill material, Box 358 is curingthe polymer epoxy. Box 360 is removing the low surface energy materiallayer. Various modifications to the above operations may be made, withcertain operations being optional. For example, in certain embodiments,the low surface energy material layer is not removed from the substrateafter the polymer is cured.

Assemblies including a substrate and die joined together as described inembodiment above may find application in a variety of electroniccomponents. FIG. 14 schematically illustrates one example of anelectronic system environment in which aspects of described embodimentsmay be embodied. Other embodiments need not include all of the featuresspecified in FIG. 14, and may include alternative features not specifiedin FIG. 14.

The system 401 of FIG. 14 may include at least one central processingunit (CPU) 403. The CPU 403, also referred to as a microprocessor, maybe a die which is attached to an integrated circuit package substrate405, which is then coupled to a printed circuit board 407, which in thisembodiment, may be a motherboard. The CPU 403 on the package substrate405 is an example of an electronic device assembly that may be formed inaccordance with embodiments such as described above. A variety of othersystem components, including, but not limited to memory and othercomponents discussed below, may also include die and substratestructures formed in accordance with the embodiments described above.

The system 401 further may further include memory 409 and one or morecontrollers 411 a, 411 b . . . 411 n, which are also disposed on themotherboard 407. The motherboard 407 may be a single layer ormulti-layered board which has a plurality of conductive lines thatprovide communication between the circuits in the package 405 and othercomponents mounted to the board 407. Alternatively, one or more of theCPU 403, memory 409 and controllers 411 a, 411 b . . . 411 n may bedisposed on other cards such as daughter cards or expansion cards. TheCPU 403, memory 409 and controllers 411 a, 411 b . . . 411 n may each beseated in individual sockets or may be connected directly to a printedcircuit board. A display 415 may also be included.

Any suitable operating system and various applications execute on theCPU 403 and reside in the memory 409. The content residing in memory 409may be cached in accordance with known caching techniques. Programs anddata in memory 409 may be swapped into storage 413 as part of memorymanagement operations. The system 401 may comprise any suitablecomputing device, including, but not limited to, a mainframe, server,personal computer, workstation, laptop, handheld computer, handheldgaming device, handheld entertainment device (for example, MP3 (movingpicture experts group layer—3 audio) player), PDA (personal digitalassistant) telephony device (wireless or wired), network appliance,virtualization device, storage controller, network controller, router,etc.

The controllers 411 a, 411 b . . . 411 n may include one or more of asystem controller, peripheral controller, memory controller, hubcontroller, I/O (input/output) bus controller, video controller, networkcontroller, storage controller, communications controller, etc. Forexample, a storage controller can control the reading of data from andthe writing of data to the storage 413 in accordance with a storageprotocol layer. The storage protocol of the layer may be any of a numberof known storage protocols. Data being written to or read from thestorage 413 may be cached in accordance with known caching techniques. Anetwork controller can include one or more protocol layers to send andreceive network packets to and from remote devices over a network 417.The network 417 may comprise a Local Area Network (LAN), the Internet, aWide Area Network (WAN), Storage Area Network (SAN), etc. Embodimentsmay be configured to transmit and receive data over a wireless networkor connection. In certain embodiments, the network controller andvarious protocol layers may employ the Ethernet protocol over unshieldedtwisted pair cable, token ring protocol, Fibre Channel protocol, etc.,or any other suitable network communication protocol.

While certain exemplary embodiments have been described above and shownin the accompanying drawings, it is to be understood that suchembodiments are merely illustrative and not restrictive, and thatembodiments are not restricted to the specific constructions andarrangements shown and described since modifications may occur to thosehaving ordinary skill in the art.

1. A device comprising: a substrate with a die attach area and an outerarea surrounding the die attach area, the substrate including outeredges, the outer area extending to the outer edges; a layer on thesubstrate in the outer area outside of the die attach area, the layerextending to the outer edges of the substrate, the layer comprising afluoropolymer material; a die coupled to the substrate in the die attacharea, wherein a gap remains between the die and the die attach area; andan underfill material in the gap and adjacent to the layer on thesubstrate; wherein the layer is positioned so that the underfillmaterial does not extend outward on the substrate from the gap betweenthe die and the substrate on at least one side of the gap between thedie and the substrate.
 2. The device of claim 1, wherein the layercomprises perfluoroalkoxy polymer resin (PFA).
 3. The device of claim 1,wherein the layer comprises polytetrafluoroethylene (PTFE).
 4. A devicecomprising: a substrate with a die attach area and an outer areasurrounding the die attach area, the substrate including outer edges,the outer area extending to the outer edges; a layer on the substrate inthe outer area outside of the die attach area, the layer extending tothe outer edges of the substrate, the layer not being positioned in thedie attach area on the substrate, the layer comprising a fluoropolymermaterial; a die coupled to the substrate in the die attach area, whereina gap remains between the die and the die attach area; and an underfillmaterial in the gap and adjacent to the layer on the substrate; whereinthe layer is positioned so that the underfill material does not extendoutward on the substrate from the gap between the die and the substrate.5. The device of claim 4, wherein the fluoropolymer material is selectedfrom the group consisting of polytetrafluoroethylene (PTFE) andperfluoroalkoxy polymer resin (PFA).
 6. A device comprising: a diecoupled to a substrate; the substrate including outer edges, the diepositioned on a die attach area spaced away from the outer edges; a gapbetween the die and the substrate; an underfill material in the gapbetween the die and the substrate; and a layer on the substrate, thelayer positioned outside of the die attach area on the substrate, thelayer not being positioned in the die attach area on the substrate, thelayer extending to the outer edges of the substrate, the layercomprising a material having a surface energy that inhibits theunderfill material from wetting the layer, the material comprising apolymer; wherein the layer is positioned so that the underfill materialdoes not extend outward on the substrate from the gap between the dieand the substrate.
 7. The device of claim 6, wherein the materialcomprises polytetrafluoroethylene (PTFE).
 8. The device of claim 6,wherein the material comprises perfluoroalkoxy polymer resin (PFA).
 9. Adevice comprising: a die coupled to a substrate; the substrate includingouter edges, the die positioned on a die attach area spaced away fromthe outer edges; a gap between the die and the substrate; an underfillmaterial in the gap between the die and the substrate; and a layer onthe substrate, the layer positioned outside of the die attach area onthe substrate, the layer not being positioned in the die attach area onthe substrate, the layer extending to the outer edges of the substrate,the layer comprising a material having a surface energy that inhibitsthe underfill material from wetting the layer, the material comprising apolymer; wherein the layer is positioned so that the underfill materialdoes not extend outward on the substrate from the gap between the dieand the substrate on at least one side of the gap between the die andthe substrate.
 10. The device of claim 9, wherein the material comprisesa fluoropolymer.
 11. The device of claim 9, wherein the die is coupledto the substrate through a plurality of solder bumps.